The present invention relates to plasma arc torches and, more particularly, to a method and apparatus for supplying a gas flow for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metal, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to a workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc in a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein, which supports the arc. The emissive element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of this low work function, the element is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and alloys thereof. A nozzle surrounds the discharge end of the electrode and provides a pathway for directing the arc towards the workpiece.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More particularly, the emissive elements of these torches often erode below the surface of the copper holder at the discharge end. Additionally, the gas tends to rapidly oxidize the copper of the electrode that surrounds the emissive element and, as the copper oxidizes, its work function decreases. As a result, a point is reached at which the oxidized copper surrounding the emissive element begins to support the arc, rather than the emissive element. When this happens, the copper oxide and the supporting copper melt, resulting in early destruction and failure of the electrode.
In order to prevent or reduce the formation of oxidized copper surrounding the emissive element, particularly for air cooled plasma arc torches, the air is circulated rapidly about the electrode to improve heat transfer from the arc away from the electrode. A conventional method for the air to be distributed in an air cooled plasma arc torch is for the air to first be used in some fashion to cool the electrode and then to be split into separate primary and secondary flows. Typically, this is accomplished by means of a gas baffle positioned between the nozzle and the electrode for splitting the flow into the primary or cutting gas flow and the secondary or shield gas flow, which helps maintain the position of the arc. More specifically, the primary flow of the gas passes through holes in the gas baffle into a chamber defined by a primary nozzle and the electrode and is ejected by the primary nozzle, while the rest of the gas is directed out a secondary nozzle so as to surround the primary gas flow. Disadvantageously, the baffle splits the gas into the primary flow and secondary flow before the nozzle chamber, which limits the ability of the torch to transfer heat from the electrode and can reduce the speed of the torch, as discussed below.
Baffles also add to the cost and complexity of manufacturing and assembling the torch. More specifically, baffles are subject to failure and can occasionally be inadvertently omitted by an operator during assembly of the torch. Furthermore, baffles tend to become brittle over time and eventually develop cracks, which often lead to catastrophic failure unless the baffles are frequently replaced. Even when replaced on a regular preventative maintenance cycle, which adds further cost to the torch, human error may lead to the baffles being left out during assembly of the torch, which can damage the torch or cause the torch to operate incorrectly. In addition, baffles can also permit the arc to xe2x80x9cjumpxe2x80x9d or track across the baffle, which can also damage the torch. Specifically, the use of baffles can result in a convoluted set of passages in and around the electrode through which air can pass, which can lead to migration of the arc through the passages. Although attempts have been made to insulate the labyrinth of passages through the torch, arcing through the often damp air in the passages has been a problem with conventional torches.
Another problem with conventional torches is the lack of cooling achieved by the gas due to splitting the gas into different flows before the gas has circulated along substantially the entire length of the electrode. In particular, many torches split the gas into the primary and secondary flows at a location intermediate the opposite ends of the electrode. This is considered necessary in order to limit the pressure realized in the nozzle chamber while providing adequate flow for cooling. In order to cool the torch while avoiding failure of the torch due to excessive nozzle pressure, often as much as 70-90% of the total gas supplied to a conventional torch is diverted away from the nozzle chamber to other outlets, which direct the secondary flow. As a result, only a portion of the total gas supplied to the torch is available for cooling the electrode along substantially the entire length of the electrode, and even less gas pressure than is optimal may be available at the exit end of the nozzle as a primary gas flow. Accordingly, conventional torches have limited cutting speeds, which adds time and expense to the torch operation. It is desirable to provide a greater nozzle chamber pressure so that higher cutting speeds can be realized. This is a difficult proposition, however, due to the limitations of conventional torches as described, and for the fact that most manufacturing locations and welding shops use standard xe2x80x9cshopxe2x80x9d air pressure that cannot be increased in order to increase the gas pressure in the nozzle chamber.
Several patents discuss plasma arc torches having various flow patterns. For example, U.S. Pat. No. 5,726,415 to Luo et al. discloses a plasma are torch with an electrode having an metallic holder with an emissive element positioned at a discharge end thereof. The torch also includes a nozzle, which in combination with the holder defines an annular gas chamber therebetween for directing a cooling gas about the electrode. The nozzle also defines a cylindrical exhaust port for directing a primary gas flow towards a workpiece, and bleed ports positioned in the rear portion of the nozzle for venting a majority of the gas through bores for use as a shield or secondary gas flow. In operation, the bleed ports bleed approximately 90% of the gas, thus leaving 10% of the gas to cool the full length of the electrode and exit the cylindrical exhaust port as the primary gas flow towards the workpiece. Thus, only a fraction of the gas entering the torch travels substantially the length of the electrode, which decreases the cooling capability of the gas.
U.S. Pat. No. 4,558,201 to Hatch discloses a plasma arc torch having a reversible electrode that has both a forward insert and a rearward insert positioned at opposing ends thereof. The electrode defines a plurality of passageways for directing the gas towards a workpiece. In particular, gas is directed through channels around the exterior of the electrode as well as through a central passage extending along the longitudinal axis of the electrode. As the gas reaches the midpoint of the electrode, however, the gas is split into a primary flow and a secondary flow, wherein the secondary flow is directed away from the electrode around an insulator to a front portion of a chamber defined by a nozzle and the insulator. The primary flow is directed out a central orifice in the nozzle along with the electrical arc extending from the forward emissive insert to the workpiece. As in the Luo ""415 patent, the gas flow is split into a primary flow and a secondary flow before the gas has traveled substantially the length of the electrode, which decreases the heat transfer capability of the gas and provides less gas pressure in the nozzle chamber, which decreases the efficiency of the torch.
Thus, there is a need to provide sufficient gas flow to the torch in order to transfer heat away from the arc and the torch, but without sacrificing cutting speed or pressure realized in the nozzle chamber. It is also desirable to provide a torch with simple assembly and without using baffles to direct a flow of gas from the electrode to the nozzle chamber.
The above and other objects and advantages of the present invention are achieved by a plasma arc torch that directs a flow of gas along substantially the length of the electrode such that more gas is used to cool the torch compared to conventional torches. The torch of the present invention includes an electrode defining a plurality of openings positioned proximate the front end of the electrode such that all of the gas supplied to the torch is directed through the openings into a chamber defined by the electrode and the nozzle. In this regard, the gas pressure in the nozzle chamber is increased compared to conventional torches, which allows the torch of the present invention to have a faster cutting speed. Advantageously, the torch of the present invention utilizes the openings in the electrode itself to direct the flow of gas, and not baffles as in conventional torches.
In particular, a plasma arc torch according to one embodiment of the present invention includes an electrode having an upper tubular member defining an internal bore and a lower cup-shaped member or holder defining a central passageway in fluid communication with the internal bore of the upper tubular member. The front end of the holder defines a cavity for receiving an emissive insert, and the rear end defines the central passageway. The holder also defines a plurality of side openings that are in fluid communication with the central passageway. In one embodiment, the side openings are arranged to impart a swirling motion to the gas flowing therethrough.
The plasma arc torch also includes a nozzle positioned proximate the front end of the holder such that a nozzle chamber is defined therebetween. The nozzle defines a central bore for discharging a primary flow of gas towards a workpiece, and in one embodiment also defines a plurality of secondary openings for creating a secondary flow of gas therethrough. Advantageously, the openings defined in the nozzle and holder eliminate the need for separate baffles for separating the gas flow into the primary and secondary flows.
Safety items are also a part of the torch of the present invention. More specifically, in one embodiment a ball valve assembly is located in the internal bore of the upper tubular member of the electrode for regulating gas flow through the electrode. In this regard, the ball valve assembly acts to protect the torch from damage if a user attempts to operate the torch with portions of the torch missing, such as the holder of the electrode, by cutting off the gas flow through the torch. The plasma arc torch of the present invention can also include a pressure switch in fluid communication with the nozzle chamber. The pressure switch can disable the torch if the gas pressure in the torch, such as in the nozzle chamber, is below a predetermined value, which may occur if the torch is assembled incorrectly or if the torch is damaged.
Methods are also a part of the present invention. According to one method of the present invention, a electrode having a metallic holder is provided, wherein the holder defines a plurality of side openings and a central passageway in fluid communication therewith. A nozzle is positioned proximate the holder to define a nozzle chamber therebetween. A flow of gas is directed through the central passageway into the nozzle chamber such that all of the gas supplied into the central passageway is directed through the side openings into the nozzle chamber. Advantageously, the gas is split into at least a primary flow and a secondary flow after the flow of gas has entered the nozzle chamber, which provides greater pressure in the nozzle chamber and allows for greater cutting speeds. To improve the ability of the torch to transfer heat from the arc, the flow of gas is directed through the central passageway a distance more than xc2xd the length of the holder before being directed through the side openings thereof. As such, more gas is available for transferring heat from the arc and electrode away from the torch.
As mentioned above, the flow of gas can also be directed to certain safety devices, such as the flow-regulating ball valve assembly, or to the pressure switch that is in fluid communication with the nozzle chamber. Advantageously, the torch is disabled if certain conditions occur, such as having a gas pressure in the torch that is below a predetermined value.
Accordingly, the present invention provides a plasma arc torch that overcomes the disadvantages of conventional torches without sacrificing the cutting speed of the torch or the pressure realized in the nozzle chamber. Advantageously, the torch and methods of the present invention avoid the use of baffles to direct the flow of gas from the electrode to a primary flow and a secondary flow, which improves the assembly, reliability, and cost of the torch.